Producer Pete Smith has confirmed LittleBigPlanet will feature around 50 levels on the disc. The game will feature an interlinking story so it will not just be a random selection of levels, although the first half of the game is just a big tutorial which introduces you to all the gameplay mechanics of the game.

Apparently, it can be completed in as little as 12 hours...

To quote:

How many levels will be on the disc?

Around 50-ish. There’ll be an interlinking story, too, so it’s not just a random selection of levels. About the first half of the game is one long tutorial, introducing you to all the gameplay mechanics in the game.

Will there be any levels that require the Create mode to get to the end?

No, not to beat the level. Though there are levels where using the create mode will allow you to access hidden areas. The Create mode will have its own set of tutorials to get you used to it. More PlayStation 3 News...